Selection for chloroquine-sensitive Plasmodium falciparum by wild Anopheles arabiensis in Southern Zambia

Background: The emergence of parasite drug resistance, especially Plasmodium falciparum, persists as a major obstacle for malaria control and elimination. To develop effective public health containment strategies, a clear understanding of factors that govern the emergence and spread of resistant parasites in the field is important. The current study documents selection for chloroquine-sensitive malaria parasites by wild Anopheles arabiensis in southern Zambia. Methods: In a 2,000-sq km region, mosquitoes were collected from human sleeping rooms using pyrethrum spray catches during the 2006 malaria transmission season. After morphological examination and molecular confirmation, vector mosquitoes were dissected to separate head and thorax from the abdominal section, followed by PCR screening for P. falciparum infection. Human residents of all ages were tested for P. falciparum parasitaemia by microscopy and PCR. Plasmodium falciparum infections were genotyped at the chloroquine resistance-conferring amino acid codon 76 of the PfCRT gene, using PCR and restriction enzyme digestion. Results: In the human population there was nearly 90% prevalence of the chloroquine-resistant PfCRT K76T mutant, with no significant differences in polymorphism among smear-positive and smear-negative (submicroscopic) infections (p = 0.323, n = 128). However, infections in both abdominal and salivary gland phases of the An. arabiensis vector exhibited wild type K76-bearing parasites with up to 9X higher odds (OR (95% CI): 9 (3.7-20.2), p < 0.0005, n = 125), despite having been acquired from humans within a few weeks. Conclusions: Anopheles arabiensis selects for wild-type K76-bearing P. falciparum during both abdominal and salivary gland phases of parasite development. The rapid vectorial selection, also recently seen with antifolate resistance, is evidence for parasite fitness cost in the mosquito, and may underpin regional heterogeneity in the emergence, spread and waning of drug resistance. Understanding the nature and direction of vector selection could be instrumental for rational curtailment of the spread of drug resistance in integrated malaria control and elimination programmes.